1. Technical Field
This invention generally relates to an improved food storage structure of double wall construction having a between wall controlled atmosphere for purposes of controlling the surface temperature of the interior wall. More particularly it relates to a double wall construction system having a forced air heating and/or cooling system within a between wall air cavity for purposes of controlling the surface temperature of the interior wall in food storage buildings requiring relatively cool, high humidity, environments for food storage.
2. Background Art
There are a number of agricultural food products such as potatoes, onions, radishes, carrots, ginger root, artichokes, etc. which are placed into bulk storages after harvest and held from a few months to up to a year prior to processing. In general, the storage environment most suitable for holding these products with a minimum of rot, shrinkage, or other losses, is a relatively cool and very humid environment. For purposes of this disclosure, the potato and potato storage parameters are used as an example, however it should be noted that the problems associated with potato storage environments, and the principals of the present invention, are equally applicable to the storage of onions, carrots, radishes, and other food products requiring a cool and humid storage environment.
The potato tuber, when harvested from the ground, is a living, respirating, organism. The potatoes are dug from the ground and hauled, in trunks, in bulk, to a storage facility where they are piled, by means of movable conveyor belts, into piles 14 feet to 22 feet high atop of ventilation pipes or ducts as shown in FIG. 1. The typical potato storage in use today ranges in size from 5,000 square feet of earthen or concrete floor space to 65,000 square feet of earthen or concrete floor space, and will hold between 2,500 tons to 40,000 tons of potatoes. Since these potatoes are living, respirating organisms, there is approximately 2 BTU's per ton of potatoes, per day, of heat generated by the dormant potatoes, or, between 5,000 BTU's to 80,000 BTU's per day of heat generated. Thus, in temperate weather conditions, the problem is not one of keeping the potatoes warm, but rather one of keeping them cool.
In general, there are two means of cooling potatoes in common use today. The first, is generally described of the ventilation system, which uses outside ambient air and is suitable for holding potatoes in storage until late spring when the thermal load on the building from the potatoes within, and the outside atmosphere, increases to the point where a suitable storage environment can no longer be maintained. Where storages are designed to hold potatoes into hot summer months, refrigeration systems are added to the ventilation to provide a supplemental cooling.
The ventilation system, as is shown in FIG. 1, consists of a primary air recirculation system wherein cool humid air is blown into ventilation ducts positioned at the bottom of the potato pile. Air coming through the ventilation ducts escapes through vent holes in the pipes and filters up through the pile of potatoes into a return air plenum or is discharged directly to the outside atmosphere through exhaust dampers. The return air plenum usually has two discharge louvers, one to the outside atmosphere to exhaust hot air, and the second a recirculation louver which returns the air to the ventilation fans in the supply air plenum. Also provided is an outside air intake louver which can also be used to supply cool, fresh air to the ventilation fans. A control system is provided which monitors the discharge air temperature atop the potato pile in the vicinity of the return air louvers and a second sensor monitors outside air temperature. Thus, in relatively moderate temperature conditions, when the outside air temperature is below the desired storage temperature, the outside air louver is opened, either partially or fully, to provide a mixture of cool outside air with warmer inside air in the return air plenum, to provide potatoes with desired air temperature and control temperature within the storage.
For longer term storage, refrigeration systems are provided to cool the return air and the outside air intake louvers are closed.
These conventional potato storage systems work relatively well in moderate weather conditions. However, during extreme conditions, either hot or cold, the storage temperature parameters cannot always be maintained in the one to five feet adjacent to the outside walls of the storage.
As previously stated, for purposes of this disclosure the storage of potatoes is used as an example. Potatoes, depending upon their intended use, either for seed, for sale as fresh produce, or for processing into potato products, will require slightly different temperature and humidity parameters, with seed potatoes being stored at between 38.degree. F. and 42.degree. F., fresh potato product being stored between 42.degree. F. and 46.degree. F. and potatoes intended for use as frozen potato products or other processed potato products, being stored between 44.degree. F. and 55.degree. F. In all cases humidity in the storage, in order to minimize shrinkage, must be maintained above 90%, but under no circumstances at a saturated humidity wherein condensation occurs. There are a number of well known methods of injecting water vapor to elevate the humidity in the storage, with the most common being the use of air washers or centrifugal humidifiers which produce a vapor fog in the supply air plenum.
The reason why condensation in the potato pile is harmful relates to the problem of rot. Each potato when dug from the ground has on its skin, a culture of various microbes such as a bacteria and yeast. These organisms are relatively harmless to the stored potatoes as long as they remain on the surface of the skin, and, temperatures are maintained in the 38.degree. F. to 55.degree. F. range. However, during the harvesting process it is inevitable that a certain percentage of the potatoes harvested will be cut and bruised, thus breaking the surface skin of the potato and exposing the interior potato pulp to the microbial population, thus allowing the microbial population to multiply, causing potato rot. The rotting of one or two isolated potatoes, in and of itself, is not of any economic significance, however, condensing water vapor, dripping down through the potato pile from the ceiling and interior walls of the storage, provides a transport mechanism for transporting cultures of rot organisms from one potato to another, thus spreading the rot within the pile. Eventually, the softer, rotting potatoes will compress one against the other, thus restricting cooling air flow, and the result will be an explosion of the microbial population and a rapid spread of the rot throughout the pile of potatoes, thus forming what is in the industry is termed a "not spot" in the potato pile.
These hot spots are very difficult to detect, if for example they were to occur 7 feet down into the potato pile, 50 feed back from the access door. As a result, potatoes in storage are periodically monitored from an inspection catwalk located in the return air plenum, wherein the farmer or processor periodically monitors the local air temperature of the air exhausting from the top of the potato pile, and potato tuber pulp temperatures at a number of different locations throughout the storage. If a severely elevated exhaust temperature or pulp temperature is detected at any point in the pile, the storage must be immediately emptied to prevent loss of the crop.
As previously stated, the transmission of rot organisms from one potato to another is generally caused by condensate water dripping down through the pile. The elimination of condensation inside the storage is a difficult problem, and the solution of which is the primary object of this invention. At 46.degree. F., and 96% of relative humidity, the dew point within the potato storage is 45.5.degree. F., just 0.5.degree. F. below the desired storage temperature, thus, if any surface temperature in the storage drops 0.5.degree. F. below the interior atmosphere temperature, condensation on the walls and ceiling will occur and condensate water will drip onto and through the potato pile. Using conventional construction techniques, condensation on the interior surfaces of the potato storage building is a virtual certainty during cold winter months.
The typical potato storage construction technique in use today is to erect a post and beam or girder framework of steel or wood which is then sheathed with corrugated panels of steel. Urethan foam insulation is then sprayed onto the interior surface to provide an insulation and vapor barrier between the interior of the storage facility and the outside atmosphere. This can generally be described as a passive wall system wherein there will always be a temperature gradient and heat loss from the interior of the building to the outside atmosphere during conditions when the outside air temperature is lower than the inside air temperature. The temperature of the interior wall and ceiling surface will generally drop below the dew point when the outside atmosphere temperature is more than 20.degree. F. below the inside storage temperature.
In addition, during prolonged, extreme cold weather conditions, the heat generated by the stored potatoes may be insufficient to maintain the interior storage temperature at the desired level in a steady state condition. In general, such extreme cold conditions are short term transients and the latent heat in the potato pile will hold the bulk of the potato pile at an acceptable storage temperature. However, it is not uncommon for those potatoes piled within a few feet of the exterior walls, to cool significantly below the desired storage temperature, and even freeze during extreme cold weather conditions. In the typical potato storage bin having dimensions of 120 feed long by 60 feet wide, with potatoes piled 16 feet high, within the storage, loss of the potatoes within 24 inches of the exterior walls will result in the loss of approximately 4% of the stored crop. This loss can occur even if the potatoes do not freeze. As in our example, if potatoes held for future processing into frozen french fires, are chilled below 36.degree. F., the dormant potatoes will start converting stored starches to sugar for use as a fuel to generate heat to maintain proper temperature. Potatoes containing unacceptably high sugar contents will produce dark brown french fries due to the carmelization of the sugar during the frying process, and thus will not be suitable for processing. Obviously potatoes that are frozen in storage are not suitable for any use, and must be discarded.
At the other extreme, during unseasonably warm weather, the thermal load on the exterior of the building will heat the interior wall and will elevate the temperature in the vicinity of the interior wall to a point where the microbial cultures on the surfaces of the potatoes will becomes active and the populations will dramatically increase, thus initiating rot at the outside edges of the potato pile. Equivalent losses can be sustained from unseasonable warm weather as those from extreme cold temperatures.
Thus, the secondary objects of the present invention are to prevent the loss of stored potatoes from temperature variations due to extreme weather conditions, either cold or hot.
Accordingly, what is needed is a means of maintaining the interior wall surface of the food storage building above the dew point for the given storage temperature and humidity parameters during periods of time when the outside atmosphere temperature is below the desired storage temperature. Additionally, what is needed is a means of cooling the interior wall surface during periods of time when the outside air temperature is significantly above the desired interior storage temperature and humidity parameters.